Schistosomiasis infects more than 200 million people worldwide. Only one effective drug against this infection is currently in production. No vaccine currently exists. Research toward development of a vaccine against schistosomiasis is the best long-term defense against propagation of drug-resistant parasites and will permit more efficient and cost-effective mass drug treatment programs. A vaccine is feasible because partial natural immunity develops in humans, and 70-90% protection can be achieved in experimental animals using radiation attenuated larvae. Although significant effort has been made toward identifying schistosome vaccine candidates, the most promising antigens trialed by World Health Organization failed to provide the stipulated minimum of 40% protection in independent tests. This poor efficacy may have occurred because for a number of reasons, but highlight the need for novel approaches to identify and test schistosome vaccine candidate antigens. Our long-term goal is to develop an effective schistosome vaccine for humans. This proposal aims to identify molecules expressed during early migratory phases of infection. Our central hypothesis is that larval secretory and/or surface proteins are essential for successful parasite invasion and survival and are principal targets of immune elimination. This derives from our own and others observations that skin-and lung-stage larvae are most vulnerable to host immune attack. To evaluate the hypothesis, we will pursue the following Specific Aims: 1) To generate and fractionate larval parasite preparations. We shall derive secretory and surface membrane preparations corresponding to skin- and lung-stage schistosomes, and fractionate them by 2D electrophoresis or MudPIT technologies. 2) To identify and characterize larval proteins with vaccine potential. We shall select gel spots or liquid chromatography (LC) fractions for analysis by mass spectrometry to link protein to encoding DNA by database searching. 3) To validate isolated larval antigens as potential vaccine candidates. We shall express larval antigens as recombinant proteins. These or coding sequences formulated in a DNA construct will be used to immunize mice to generate antibodies that will be used to probe native protein preparations and to localize the parasite tissue of origin. Immunized mice will also be challenged to assess protective potential of vaccine candidates. In this, the first systematic study of larval antigens as vaccine candidates, we expect to identify several novel antigens that could be incorporated into a multivalent vaccine. Partial identification of the larval proteome will also help to better define host-parasite interaction in schistosomiasis. An effective vaccine could transform control programs for schistosomiasis by reducing the costs of repeated treatment, and the risk of developing drug resistance.